Question 1 · Hard
A 41-year-old camel herder in Al Ain had acute brucellosis 5 months ago, with Brucella melitensis in blood culture. He was prescribed doxycycline and rifampicin for 6 weeks but stopped after 3 weeks when he felt better. For 10 days he has again had fever, sweats and a painful knee. Standard tube agglutination (SAT) titers, with the stored samples retested in the same run: at diagnosis, SAT 1:640 and 2-mercaptoethanol (2-ME) SAT 1:80; 2 months later, SAT 1:160 and 2-ME SAT 1:40; now, SAT 1:640 and 2-ME SAT 1:640. The best interpretation is:
- AResidual antibody from the treated infection
- BActive brucellosis, most likely a relapse
- CA new IgM response from fresh exposure
- DCross-reacting antibody to Yersinia O:9
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Answer: B · Active brucellosis, most likely a relapse
SAT measures agglutinating IgM and IgG together, while 2-ME breaks the disulfide bonds that hold the IgM pentamer together, so the 2-ME titer reflects IgG. At diagnosis most of the activity was IgM, and after treatment both titers fell, but now the 2-ME-resistant titer has risen 16-fold, from 1:40 to 1:640, together with fever and arthritis: this is active infection. Stopping treatment early and the return of symptoms within 6 months make relapse the most likely cause, although serology alone cannot exclude reinfection. Residual antibody would be stable or falling, and a fresh IgM response would leave the 2-ME titer well below the SAT. Blood cultures should be repeated, with the laboratory warned that Brucella is suspected, and a full course of treatment given.
- A: Residual antibody would be stable or falling, not rising 16-fold with symptoms.
- C: All the activity survives 2-ME, so it is IgG, not a new IgM response.
- D: A cross-reaction would not explain a rising IgG titer after proven brucellosis.